This invention relates to fluoroelastomers that are capable of being crosslinked with polyhydroxy compounds to produce cured compositions having excellent processability and low temperature properties.
Elastomeric fluoropolymers (i.e. fluoroelastomers) exhibit excellent resistance to the effects of heat, weather, oil, solvents and chemicals. Such materials are commercially available and are most commonly either dipolymers of vinylidene fluoride (VF2) with hexafluoropropylene (HFP) or terpolymers of VF2, HFP, and tetrafluoroethylene (TFE). While these di- and terpolymers have many desirable properties, including low compression set and excellent processability, their low temperature flexibility is not adequate for all applications.
It is known that incorporation of perfluorinated ether monomer units into vinylidene fluoride elastomers improves low temperature properties. For example, Carlson, in U.S. Pat. No. 5,214,106 discloses that when perfluoro(methyl vinyl) ether (PMVE) is substituted for HFP, the resultant VF2/PMVE/TFE copolymers have glass transition temperature (Tg) values which are 10xc2x0-20xc2x0 C. lower than those of the corresponding VF2/HFP/TFE copolymers. Tg is often used as an indicator of low temperature flexibility because polymers having low glass transition temperatures maintain elastomeric properties at low temperatures.
Kruger, in U.S. Pat. No. 5,696,216, discloses PMVE-containing fluoroelastomers that are similar to those disclosed by Carlson. Those disclosed by Kruger contain copolymerized units of VF2; at least one fluorinated propene and or fluorinated methyl vinyl ether; TFE; at least one perfluoro(polyoxyalkyl vinyl) ether, and a crosslinking site.
The compositions of Carlson and Kruger are most effectively crosslinked through use of peroxide cure systems. However, when compression molding equipment is used with peroxide curable VF2/PMVE copolymers the compositions generally exhibit a tendency to stick to and foul the mold.
Tetrapolymers of VF2, HFP, TFE and perfluoro(alkyl vinyl) ethers (PAVE) other than PMVE are also known to exhibit improved low temperature properties compared to terpolymers of VF2, HFP and TFE. For example, Arcella, et al. in U.S. Pat. No. 5,260,393 disclose a tetrapolymer comprising copolymerized units of 48-65 wt. % VF2, 21-36 wt. % HFP, 3-9 wt. % PAVE, and 0-17 wt. % TFE. The compositions can be cured using a bisphenol curing system and do not exhibit the mold fouling problems associated with peroxide cures of VF2/PMVE copolymers. Similarly, British Patent 1,296,084 discloses fluoroelastomeric tetrapolymers containing copolymerized units of 48-65 wt. % VF2, 8-23 wt. % HFP, 4-15 wt. % TFE, and 17-30 wt. % PAVE. Such compositions have good low temperature properties and are curable with bisphenols or amines. Although these tetrapolymers exhibit good low temperature properties, many applications require improved low temperature and processability performance.
Merely raising the PAVE content while lowering the HFP content is not a solution to the problem of improving low temperature performance of VF2/HFP/PAVE/TFE terpolymers. This is because polymers wherein the level of HFP is below about 8-10 mole percent do not contain sufficient copolymerized monomer sequences consisting of HFP units flanked by VF2 units to permit efficient crosslinking with bisphenols. As is well known in the art, efficient curing of VF2/HFP-containing fluoroelastomers with a bisphenol/accelerator system is possible only when a xe2x80x94CH2xe2x80x94 group in the polymer backbone is flanked by two perfluorinated carbons (e.g. CF2CF(CF3)CH2CF2CF2), rendering the hydrogens acidic enough to be abstracted by base. The dehydrofluorinated polymers are easily crosslinked by bisphenols. Furthermore, as discussed by W. W. Schmiegel, in Angewandte Makromolekylare Chemie, 76/77, 39 (1979), completely eliminating HFP to form VF2/TFE/PMVE terpolymers results in formation of monomer sequences consisting of TFE/VF2/TFE; TFE/VF2/PMVE; PMVE/VF2/PMVE; and PMVE/VF2/TFE. Although such sites readily undergo elimination of HF or HOCF3 in the presence of base, the double bonds thus formed are not easily crosslinked by bisphenols or any other traditional crosslinking agents.
There thus exists an unfulfilled need in the art for a method of providing copolymers of VF2, TFE, and PAVE that maintain optimum low temperature properties, but which exhibit low mold sticking characteristics, improved processability and are easily curable.
The present invention is directed to a fluoroelastomer consisting essentially of copolymerized units of 23-65 weight percent vinylidene fluoride, 25-75 weight percent perfluoro(alkyl vinyl) ether, 0-30 weight percent tetrafluoroethylene, and 0.3-5 weight percent 2-hydropentafluoropropene.
In addition, the invention is directed to a curable composition comprising
A. a fluoroelastomer consisting essentially of copolymerized units of 23-65 weight percent vinylidene fluoride, 25-75 weight percent perfluoro(alkyl vinyl) ether, 0-30 weight percent tetrafluoroethylene, and 0.3-5 weight percent 2-hydropentafluoropropene;
B. a polyhydroxy crosslinking agent;
C. a cure accelerator; and
D. a metal oxide or metal hydroxide.
A preferred embodiment of the curable compositions of the invention additionally comprises a modified silane coated mineral filler.
A further preferred embodiment of the curable compositions of the invention additionally comprises a molecular sieve.
The polymers of the present invention include both uncured (raw) and cured fluorinated copolymers. The copolymers are capable of undergoing crosslinking reactions with polyhydroxylic compounds to form elastomeric compositions that exhibit unusually good low temperature properties.
The polymer backbones of the copolymers consist essentially of copolymerized units of VF2, PAVE, 2-hydropentafluoropropene (i.e. 1,1,3,3,3-pentafluoropropene), referred to herein as HPFP, and, optionally, TFE. That is, each of the first three monomers (and optionally TFE) must be present in the polymer chain, but higher order polymers, i.e. those containing other additional monomer units, the addition of which does not affect the basic and novel characteristics of the polymer, are also within the scope of the present invention. For example, the tetrapolymer VF2/PAVE/TFE/HPFP can contain other copolymerized vinyl or olefin monomers such as vinyl fluoride, trifluoroethylene, trifluoropropene, chlorotrifluoroethylene, alkyl vinyl ether, vinyl acetate, vinyl chloride, ethylene, and propylene, generally in quantities of up to about 5 wt. %. In addition, the fluoroelastomer copolymers of this invention may contain up to about 1 wt. % iodine bound to polymer chain ends, the iodine being introduced via use of an iodine-containing chain transfer agent during polymerization.
The fluoroelastomers of the invention contain between 23-65 wt. % copolymerized vinylidene fluoride units, preferably between 33-55 wt. % of such units. If less than 23 wt. % vinylidene fluoride units are present, the polymerization rate is very slow. In addition, good low temperature flexibility cannot be achieved. Vinylidene fluoride levels above 65 wt. % result in polymers that contain crystalline domains and are characterized by poor low temperature compression set resistance and reduced fluids resistance.
Perfluoro(alkyl vinyl) ethers (PAVE) suitable for use as comonomers include those of the formula
CF2xe2x95x90CFO(Rfxe2x80x2O)n(Rfxe2x80x3O)mRfxe2x80x83xe2x80x83(I) 
where Rfxe2x80x2 and Rfxe2x80x3 are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and Rf is a perfluoroalkyl group of 1-6 carbon atoms.
A preferred class of perfluoro(alkyl vinyl) ethers includes compositions of the formula
CF2xe2x95x90CFO(CF2CFXO)nRfxe2x80x83xe2x80x83(II) 
where X is F or CF3, n is 0-5, and Rf is a perfluoroalkyl group of 1-6 carbon atoms.
A most preferred class of perfluoro(alkyl vinyl) ethers includes those ethers wherein n is 0 or 1 and Rf contains 1-3 carbon atoms. Examples of such perfluorinated ethers include perfluoro(methyl vinyl) ether and perfluoro(propyl vinyl) ether. Other useful monomers include compounds of the formula
CF2xe2x95x90CFO[(CF2)mCF2CFZO]nRfxe2x80x83xe2x80x83(III) 
where Rf is a perfluoroalkyl group having 1-6 carbon atoms,
m=0 or 1, n=0-5, and Z=F or CF3.
Preferred members of this class are those in which Rf is C3F7, m=0, and n=1.
Additional perfluoro(alkyl vinyl) ether monomers include compounds of the formula
CF2xe2x95x90CFO[(CF2CFCF3O)n(CF2CF2CF2O)m(CF2)p]CxF2x+1xe2x80x83xe2x80x83(IV) 
where m and n independently=1-10, p=0-3, and x=1-5.
Preferred members of this class include compounds where n=0-1, m=0-1, and x=1.
Examples of useful perfluoro(alkoxy vinyl) ethers include
CF2xe2x95x90CFOCF2CF(CF3)O(CF2O)mCnF2n+1xe2x80x83xe2x80x83(V) 
where n=1-5, m=1-3, and where, preferably, n=1.
Mixtures of perfluoro(alkyl vinyl) ethers and perfluoro(alkoxy vinyl) ethers may also be used.
The perfluoro(alkyl vinyl) ether content of the fluoroelastomers of the invention ranges from 25-75 wt. %. If perfluoro(methyl vinyl) ether is used, then the fluoroelastomer preferably contains between 30-44 wt. % copolymerized perfluoroether units. If less than 25 wt. % perfluoro(alkyl vinyl) ether is present, the low temperature properties of the fluoroelastomers are adversely affected.
Copolymerized units of tetrafluoroethylene may also be present in the fluoroelastomers of the invention at levels up to 30 wt. %. The presence of copolymerized units of TFE is desirable for the purpose of increasing fluorine content without unduly compromising low temperature flexibility. High fluorine content promotes good fluid resistance. If TFE is present as a comonomer, it is preferably copolymerized in amounts of at least 3 wt. %. Levels of 3 wt. % or greater TFE lead to improved fluid resistance in some end use applications. TFE levels above 30 wt. % result in some polymer crystallinity which affects low temperature compression set and flexibility.
The fourth copolymerized monomer unit in the polymers of the invention is 2-hydropentafluoropropene (HPFP). A particular characteristic of the HPFP monomer is that it acts as an independent cure site monomer that takes part in crosslinking reactions with polyhydroxylic curing agents. Polymers that contain copolymerized HPFP monomer units do not require the presence of copolymerized monomer sequences of VF2 flanked by perfluoromonomers (e.g. HFP/VF2/HFP) for initiation of dehydrofluorination. Introduction of copolymerized HPFP units into the VF2/HFP copolymer chain creates sites that exceed the reactivity of HFP/VF2/HFP sequences. HFP is a perfluorinated monomer and thus contains no hydrogens. It cannot function as an independent cure site monomer because it is incapable of undergoing dehydrofluorination. In fact, HFP-containing VF2 copolymers of PMVE must contain at least about 8-10 wt. % HFP in order to provide a sufficient concentration of xe2x80x94CF2CF(CF3)CH2CF2CF2xe2x80x94 sequences for effective cure by polyhydroxylic compounds.
HPFP/TFE/PMVE terpolymers are disclosed in U.S. Pat. Nos. 5,478,902 and 5,719,245. In addition, HPFP/TFE/PMVE tetrapolymers containing not more than about 20 mole percent of an additional monomer are disclosed therein. Compositions containing high levels of VF2 comonomer are not disclosed. In addition, U.S. Pat. No. 5,874,506 discloses VF2/TFE/HFP/HPFP tetrapolymers. The polymers must contain 16-30 mol % HFP. Pentapolymers containing up to 5 mol % of additional comonomers are also disclosed therein. The tetrapolymers and pentapolymers disclosed in this reference do not exhibit good low temperature properties and have very different fluids resistance from the polymers of the present invention.
Because of the ease of hydrogen abstraction in HPFP-containing VF2 fluoroelastomers, the polymers of the present invention require only low levels of HPFP, i.e. 0.3-5 wt. %, to promote efficient polyhydroxylic cures. This permits adjustment of other comonomer levels to maximize particular physical properties. Thus, the polymers of the present invention exhibit excellent cure characteristics with only low levels of HPFP. They maintain the high temperature compression set resistance properties and excellent cure response characteristic of polymers having significant amounts of copolymerized VF2. Further, they exhibit a combination of excellent low temperature properties and processability not found in prior art fluoroelastomers. Preferably levels of HPFP will be between 0.7 and 3.0 wt. %.
The polymers of this invention may be prepared using free radical batch or semi-batch, or continuous free radical emulsion polymerization processes. They may also be prepared by free radical suspension polymerization processes.
For example, if a continuous emulsion process is utilized, the polymers are generally prepared in a continuous stirred tank reactor. Polymerization temperatures may be in the range of 40xc2x0 to 145xc2x0 C., preferably 100xc2x0 to 135xc2x0 C. at pressures of 2 to 8 MPa. Residence times of 20 to 60 minutes are preferred. Free radical generation may be effected through use of a water-soluble initiator such as ammonium persulfate, either by thermal decomposition or by reaction with a reducing agent such as sodium sulfite. An inert surface-active agent such as ammonium perfluorooctanoate may be utilized to stabilize the dispersion, usually in conjunction with addition of a base such as sodium hydroxide or a buffer such as disodium phosphate to control pH in the range 3 to 7. Unreacted monomer is removed from the reactor effluent latex by vaporization at reduced pressure. Polymer is recovered from the stripped latex by coagulation. For example, coagulation may be effected by reducing latex pH to about 3 by addition of acid, then adding a salt solution, such as an aqueous solution of calcium nitrate, magnesium sulfate, or potassium aluminum sulfate, to the acidified latex. The polymer is separated from the serum, then washed with water and subsequently dried. After drying, the product may be cured.
Chain transfer agents may be used in the polymerization in order to control the molecular weight distribution of the resulting polymers. Examples of chain transfer agents include isopropanol; methyl ethyl ketone; ethyl acetate; diethyl malonate; isopentane; 1,3-diiodoperfluoropropane; 1,4-diiodoperfluorobutane; 1,6-diiodoperfluorohexane; 1,8-diiodoperfluorooctane; methylene iodide; trifluoromethyl iodide; perfluoro(isopropyl) iodide; and perfluoro(n-heptyl) iodide. Polymerization in the presence of iodine-containing chain transfer agents may result in a polymer with one or two iodine atoms per fluoroelastomer polymer chain, bound at the chain ends (see for example U.S. Pat. No. 4,243,770 and U.S. Pat. No. 4,361,678). Such polymers may have improved flow and processability compared to polymers made in the absence of a chain transfer agent. Generally, up to about 1 weight percent iodine chemically bound to fluoroelastomer chain ends will be incorporated into the polymer, preferably from 0.1-0.3 wt. %.
Another embodiment of the present invention is a curable composition that comprises the above-described copolymers and a polyhydroxylic curing agent. The polymers of the invention are also curable with amines and amine derivatives, for example carbamates.
Any of the known polyhydroxylic aromatic crosslinking agents that require accelerators for satisfactory cure rates are suitable for use with the fluoroelastomers of the present invention. The crosslinking agent is usually added in amounts of from about 0.5-4 parts by weight per hundred parts by weight fluoroelastomer (phr), usually 1-2.5 phr. Preferred crosslinking agents are di- tri-, tetrahydroxybenzenes, naphthalenes, anthracenes and bisphenols of the formula 
where A is a stable divalent radical, such as a difunctional aliphatic, cycloaliphatic, or aromatic radical of 1-13 carbon atoms, or a thio, oxy, carbonyl, sulfinyl, or sulfonyl radical; A is optionally substituted with at least one chlorine or fluorine atom; x is 0 or 1; n is 1 or 2 and any aromatic ring of the polyhydroxylic compound is optionally substituted with at least one atom of chlorine, fluorine, or bromine, a xe2x80x94CHO group, or a carboxyl or acyl radical (e.g. a xe2x80x94COR where R is OH or a C1-C8 alkyl, aryl, or cycloalkyl group). It will be understood from the above formula describing bisphenols that the xe2x80x94OH groups can be attached in any position (other than number one) in either ring. Blends of two or more such compounds can also be used.
Referring to the bisphenol formula shown in the previous paragraph, when A is alkylene, it can be, for example, methylene, ethylene, chloroethylene, fluoroethylene, difluoroethylene, 1,3-propylene, 1,2-propylene, tetramethylene, chlorotetramethylene, fluorotetramethylene, trifluorotetramethylene, 2-methyl -1,3-propylene, 2-methyl-1,2-propylene, pentamethylene, and hexamethylene. When A is alkylidene, it can be for example ethylidene, dichloroethylidene, difluoroethylidene, propylidene, isopropylidene, trifluoroisopropylidene, hexafluoroisopropylidene, butylidene, heptachlorobutylidene, heptafluorobutylidene, pentylidene, hexylidene, and 1,1-cyclohexylidene. When A is a cycloalkylene radical, it can be for example 1,4-cyclohexylene, 2-chloro-1,4-cyclohexylene, 2-fluoro-1,4-cyclohexylene, 1,3-cyclohexylene, cyclopentylene, chlorocyclopentylene, fluorocyclopentylene, and cycloheptylene. Further, A can be an arylene radical such as m-phenylene, p-phenylene, 2-chloro-1,4-phenylene, 2-fluoro-1,4-phenylene, o-phenylene, methylphenylene, dimethylphenylene, trimethylphenylene, tetramethylphenylene, 1,4-naphthylene, 3-fluoro-1,4-naphthylene, 5-chloro-1,4-naphthylene, 1,5-naphthylene, and 2,6-naphthylene.
Other useful crosslinking agents include hydroquinone, dihydroxybenzenes such as catechol, resorcinol, 2-methyl resorcinol, 5-methyl resorcinol, 2-methyl hydroquinone, 2,5-dimethyl hydroquinone; 2-t-butyl hydroquinone; and 1,5-dihydroxynaphthalene.
Additional polyhydroxy curing agents include alkali metal salts of bisphenol anions, quaternary ammonium salts of bisphenol anions and quaternary phosphonium salts of bisphenol anions. For example, the salts of bisphenol A and bisphenol AF. Specific examples include the disodium salt of bisphenol AF, the dipotassilum salt of bisphenol AF, the monosodium monopotassium salt of bisphenol AF and the benzyltriphenylphosphonium salt of bisphenol AF. Quaternary ammonium and phosphonium salts of bisphenol anions and their preparation are discussed in U.S. Pat. Nos. 4,957,975 and 5,648,429.
In addition, derivatized polyhydroxy compounds, such as diesters, are useful crosslinking agents. Examples of such compositions include diesters of phenols, such as the diacetate of bisphenol AF, the diacetate of sulfonyl diphenol, and the diacetate of hydroquinone.
When cured with polyhydroxy compounds, the curable compositions will also generally include a cure accelerator. The most useful accelerators are quaternary phosphonium salts, quaternary alkylammonium salts, or tertiary sulfonium salts. Particularly preferred accelerators are n-tetrabutylammonium hydrogen sulfate, tributylallylphosphonium chloride and benzyltriphenylphosphonium chloride. Other useful accelerators include those described in U.S. Pat. Nos. 5,591,804; 4,912,171; 4,882,390; 4,259,463 and 4,250,278 such as tributylbenzylammonium chloride, tetrabutylammonium bromide, tetrabutylammonium chloride, benzyl tris(dimethylamino)phosphonium chloride; 8-benzyl-1,8-diazabicyclo[5,4,0]-7-undecenonium chloride, [(C6H5)2S+(C6H13)][Cl]xe2x88x92, and [(C6H13)2S(C6H5)]+[CH3CO2]xe2x88x92. In general, about 0.2 phr accelerator is an effective amount, and preferably about 0.35-1.5 phr is used.
If quaternary ammonium or phosphonium salts of bisphenols are used as curing agents, then addition of a cure accelerator is not necessary.
The polyhydroxy cure system will also contain a metal compound composed of a divalent metal oxide, such as magnesium oxide, zinc oxide, calcium oxide, or lead oxide, or a divalent metal hydroxide; or a mixture of the oxide and/or hydroxide with a metal salt of a weak acid, for example a mixture containing about 1-70 percent by weight of the metal salt. Among the useful metal salts of weak acids are barium, sodium, potassium, lead, and calcium stearates, benzoates, carbonates, oxalates, and phosphites. The amount of the metal compound added is generally about 1-15 phr, about 2-10 parts being preferred.
Diamines and diamine carbamates are also useful curing agents for the compositions of the invention. Examples of useful diamines include N,Nxe2x80x2-dicinnamylidene-1,6-hexanediamine, trimethylenediamine, cinnamylidene trimethylenediamine, cinnamylidene ethylenediamine, and cinnamylidene hexamethylenediamine. Examples of useful carbamates are hexamethylenediamine carbamate, bis(4-aminocyclohexyl)methane carbamate, 1,3-diaminopropane monocarbamate, ethylenediamine carbamate and trimethylenediamine carbamate. Usually about 0.1-5 phr of the carbamate is used.
Other additives may be compounded into the fluoroelastomer to optimize various physical properties. Such additives include carbon black, stabilizers, plasticizers, lubricants, pigments, fillers, and processing aids typically utilized in perfluoroelastomer compounding. Any of these additives can be incorporated into the compositions of the present invention, provided the additive has adequate stability for the intended service conditions.
Carbon black is used in elastomers as a means to balance modulus, tensile strength, elongation, hardness, abrasion resistance, conductivity, and processability of the compositions. Carbon black is generally useful in amounts of from 5-60 phr.
In addition, or in the alternative, fluoropolymer fillers may be present in the composition. Generally from 1 to 50 phr of a fluoropolymer filler is used, and preferably at least about 5 phr is present. The fluoropolymer filler can be any finely divided, easily dispersed plastic fluoropolymer that is solid at the highest temperature utilized in fabrication and curing of the perfluoroelastomer composition. By solid, it is meant that the fluoroplastic, if partially crystalline, will have a crystalline melting temperature above the processing temperature(s) of the perfluoroelastomer(s). Such finely divided, easily dispersed fluoroplastics are commonly called micropowders or fluoroadditives. Micropowders are ordinarily partially crystalline polymers.
A preferred additive class includes molecular sieves, particularly zeolites. Molecular sieve zeolites are crystalline aluminosilicates of Group IA and Group IIA elements, such as sodium, potassium, magnesium, and calcium. Chemically, they are represented by the empirical formula: M2/nO.Al2O3.ySiO2.wH2O where y is 2 or greater, n is the cation valence, and w represents the water contained in the voids of the zeolite. Commercially available examples of such compositions include Molecular Sieve 3A, Molecular Sieve 4A, Molecular Sieve 5A, and Molecular Sieve 13X, all available from Aldrich Chemical Co., Inc. Milwaukee, Wis. Use of this class of additives prevents sponging and improves heat aging of vulcanizates upon press curing in many instances. In general, use of about 1-5 phr is sufficient.
Other preferred additives include modified silane coated mineral fillers. By xe2x80x9cmodified silanexe2x80x9d is meant that the silane contains at least one reactive functional group such as an amino group, or an epoxy group. The mineral fillers used in this invention are preferably somewhat alkaline, such as calcium metasilicates (CaSiO3), especially wollastonite. Wollastonite coated with either an aminosilane or an epoxysilane is especially preferred. These compounds are commercially available from Quartzwerke GmbH of Freschen, Germany as Tremin(copyright)283 EST (epoxysilane treated wollastonite) and Tremin(copyright)283 AST (aminosilane treated wollastonite). These modified silane coated mineral fillers prevent sponging of the fluoroelastomer composition during press cure and also accelerate the cure rate. Generally, about 5 to 80 phr modified silane coated mineral filler is useful in the compositions of this invention, about 10 to 60 phr being preferred.
Organotin hydrides are another class of additive that may be employed. Tri-n-butyltin hydride (TBTH) is especially preferred. These tin hydride fillers accelerate the cure rate of the compositions of this invention and increase the modulus and improve the compression set resistance of the cured compounds. Generally, about 0.2 to 1.5 phr organotin hydride filler is useful, about 0.4 to 0.8 phr being preferred.
The crosslinking agent, accelerator, metal oxide, and other additives are generally incorporated into the polymer by means of an internal mixer or on a rubber mill. The resultant composition is then cured, generally by means of heat and pressure, for example by compression transfer or injection molding.
The curable compositions of the present invention are useful in production of gaskets, tubing, seals and other molded components. Such articles are generally produced by molding a compounded formulation of the curable composition with various additives under pressure, curing the part, and then subjecting it to a post cure cycle. The cured compositions have excellent low temperature flexibility and processability as well as excellent thermal stability and chemical resistance. They are particularly useful in applications such as seals and gaskets requiring a good combination of oil resistance, fuel resistance and low temperature flexibility, for example in fuel injection systems, fuel line connector systems and in other seals for high and low temperature automotive uses.
The invention is now illustrated by certain embodiments wherein all parts and percentages are by weight unless otherwise specified.